This invention relates to a linear-movement potentiometer and more particularly to a linear-movement potentiometer particularly suitable for use in a steering torque detecting unit of an automotive power steering system.
FIG. 1 is a sectional view showing a conventional linear-movement potentiometer to which the present invention pertains. In FIG. 1, reference numeral 1 designates a caing, 2 is an operating rod, 3 is a cover, 4 is a sleeve bearing, 5 is a carrier, 6 is a substrate, 7 is a slider contact, 8 are terminals, 9 are springs, 10 are lead wires, 11 is a grommet, 12 is a guide rail, 13 is a filler resin, 14 is a vent hole, 15 is a spring and 16 is an engagement aperture.
FIG. 2 is a plan view illustrating in detail the positional relationship between the substrate 6 and the slider contact 7 shown in FIG. 1. 6a is a collector printed on the substrate 6 and 6b is a resistor also printed on the substrate. As illustrated in FIG. 2, the slider contact 7 bridges between and slidably contacts with the collector 6a and the resistor 6b.
Referring again to FIG. 1, the slider contact 7 is secured to the carrier 5 which is guided by a guide rail 12 for a linear movement therealong, and the carrier 5 is securely mounted to the operating rod 2. The operating rod 2 is connectable at an outer end through the engagement aperture 16 to a mechanism for converting the torsion angle in the torsion bar of a steering shaft between an input end (a steering wheel side) and an output end (a steering gear side) into a linear displacement. Thus, when the operating rod 2 linearly moves, and the slider contact 7 correspondingly moves, an electrical signal (voltage) proportional to the above-mentioned torsion angle can be be obtained from the lead wire 10. This voltage signal causes a steering motor connected to the steering shaft to be driven to operate the automotive power steering system.
When assembling the potentiometer, after the operating rod 2 having the carrier 5 secured thereon is inserted into the casing 1 and through the sleeve bearing 4, the substrate 6 is inserted between the springs 9 and the slider contact 7 on the carrier 5. Then, after the cover plate 3 is fitted to hold the substrate 6 by the spring 15, the filler resin 13 is poured to hold the cover plate 3 and the lead wires 10.
As the operating rod 2 moves inward and outward with respect to the casing 1, the volume of the inner space varies and the air pressure within the casing changes accordingly, whereby a pressure difference which impedes smooth movement of the operating rod 2 appears between the exterior and the interior of the casing 1. In order to prevent such a pressure difference from being generated, the bottom wall of the casing 1 is provided with the vent hole 14. Although not illustrated, instead of the vent hole 14 formed in the bottom wall of the casing 1, a relatively large clearance may be made between the operating rod 2 and the sleeve bearing 4 for allowing the air to pass therethrough.
With the above structure of the conventional potentiometer, moisture or water drops can relatively easily enter into the casing through the vent hole 14 or the clearance between the operating rod 2 and the sleeve bearing 4. The moisture entered into the casing 1 may generate erroneous electric signals when attached to the resistor 6b or generate rust on the slider or the like.